Printed circuit boards (PCB's) serve the purpose of an intermediate level of packaging for attachment and electrical inter-connection of discrete devices such as resistors and capacitors and active devices such as logic and memory modules. Printed circuit board may also have edge connectors for physical-attachment and electrical connection of the printed circuit board to a higher level of packaging. Devices are attached to printed circuit boards by a variety of methods. One attachment method is soldering to pads of discrete passive surface mount technology (SMT) components or active device ball grid array (BGA) or land grid array (LGA) modules. Another attachment method is pin in hole (PIH) soldering. PIH soldering requires printed circuit having plated through holes (PTH's). The pads and edge connectors are interconnected by wiring traces formed on the top surface of the printed circuit board. Often wiring traces are formed on the bottom surface of the board and within the board itself. Similarly, devices may be mounted on both the top and bottom surfaces of the printed circuit board.
Pads for SMT, BGA, LGA, and PIH/LGA solder connections require surface finish layers that provide good solderability and corrosion resistance. Edge connectors require surface finish layers for good electrical and mechanical contact, wear resistance, and corrosion resistance. Wiring traces must be protected from solder operations, often by a solder mask. The solder mask must have good adhesion to the wiring trace, requiring a application of a surface finish or surface treatment of the wiring trace.
Generally it is not possible for one surface finish to satisfy all the requirements thus placed on the printed circuit board so multiple surface finishes and/or surface treatments are required. Often multiple finishes require multiple masking steps. When multiple surface finishes or treatments are applied to a printed circuit board compatibility of the processes employed to produce those finishes is critical. For example, the chemicals used to remove masks should not corrode or contaminate any prior formed surface finishes. Similarly, the processes used to form a given surface finish should not corrode or contaminate other surface finishes or printed circuit board features.